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How to get it right: Notches & holes in solid timber joists

Notches and holes in solid timber joists

When installing pipework or cabling, floor or ceiling joists may need to be notched or drilled. But this could weaken the floor to such an extent that it becomes structurally unsound.

To find out if this might be the case, guidance on notches and holes in solid timber joists for use in domestic properties can be found in section 3.22 of Eurocode 5 published by BM TRADA. 

The diagrams and tables below illustrate the zones and sizes permissible for differing spans and joist depths.

Notches and holes in sold timber joists - diagram 1Notches and holes in solid timber joists

 

Span of timber Hole locations between Notching locations between
mm 0.25 of span 0.4 of span 0.07 of span 0.25 of span
2000 500 800 140 500
2500 625 1000 175 625
3000 750 1200 210 750
3500 875 1400 245 875
4000 1000 1600 280 1000
4500 1125 1800 315 1125

 

Depth of timber Max hole diameter Max notch depth
mm 0.25 x depth mm 0.125 x depth mm
100 25.0 12.5
125 31.3 15.6
150 37.5 18.8
175 43.8 21.9
200 50.0 25.0
225 56.3 28.1
250 62.5 31.3
275 65 (max) 34.4
300 65 (max) 35

Key points for notches and holes

Before you start work check that the joist size is adequate for the span.

Notches can only be made in the top OR bottom of the joists, (not both) within the permitted area to a maximum of 35mm.

Holes may only be made on the joist's centreline within the permitted area to a maximum diameter of 65mm.

Additional holes must be at least three diameters (centre to centre) apart.

Single/multiple timber structural beams, rafters, purlins and binders must never be notched or drilled without calculations to justify the remaining timber. Rafters may be birdsmouthed by no more than 1/3rd of the depth.

This guidance does not apply to engineered joists e.g. JJI joists and reference should be made to specific product manufacturers instructions.

Further information

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Published January 2018